Nitrogen oxides (NO.sub.x) are major environmental pollutants. Nitric oxides, although unstable, exhibit a high resistance to decomposition. There has as a consequence been a continuing search for catalysts capable of facilitating the decomposition of nitrogen oxides, and this search dates back to the beginning of the twentieth century.
The most active of the NO.sub.x decomposition catalysts that have been identified are noble metals and oxides of transition metals. However, it is impractical to use such catalysts in applications such as the decomposition of nitrogen oxides in post-combustion flue gases and other high temperature gas streams. One reason is that flue gases invariably contain oxygen and sulfur dioxide. Oxygen has a strong inhibiting effect on the catalytic activity of noble metals and transition metal oxides, and sulfur dioxide is a strong catalyst poison. In addition, there are typically very few locations in a system at which stack gases and other exhaust streams can be treated as a practical matter. In a utility boiler system, for example, there are only two convenient locations for a nitrogen oxide-decomposing catalytic reactor, one after the economizer and the other in the path between the air preheater and the exhaust stack inlet. The respective temperatures of the gases at these locations are approximately 350.degree.-450.degree. C. and 100.degree.-150.degree. C. At these temperatures, noble metal and transition metal oxide catalysts are not active enough to be of practical value.
Selective catalytic reduction (SCR) technology has been widely accepted for the post-economizer, 350.degree.-450.degree. C. decomposition of nitrogen oxides. In this technology, ammonia is used as a reducing agent with a V.sub.2 O.sub.5 /TiO.sub.2 catalyst or a WO.sub.3 +V.sub.2 O.sub.5 /TiO.sub.2 catalyst.
The low temperature window (100.degree.-150.degree. C.) in the air preheater/stack inlet path presents an economically more attractive alternative for the selective catalytic reduction of nitrogen oxides; and catalysts usable in low temperature SCR processes have been found. These include supported platinum, iron nickel sulfates, amorphous chromia, Nb.sub.2 O.sub.5 +V.sub.2 O.sub.5 /TiO.sub.2, and manganese oxides. However, because of the low concentrations of nitrogen oxides in the settings which they are found, the direct, catalytically promoted decomposition of nitrogen oxides at mild temperatures (below 600.degree. C.) without the use of a reducing gas results in low decomposition rates. As a consequence, and despite the large number of catalysts that have been found to decompose nitrogen oxides, efforts to directly decompose those pollutants without using reducing gases have been essentially abandoned.
Consequently, there is an existing and continuing need for a nitrogen oxide decomposition process which overcomes the problems posed by the low concentrations in which polluting nitrogen oxides are typically found.